Conventional snowmobile powertrains incorporate a continuously variable transmission (CVT) having a drive pulley that is operatively coupled to the engine crankshaft and a driven pulley coupled to a driven shaft. The drive pulley transfers torque to the driven pulley via a drive belt looped around both pulleys. Typically, the driven shaft is a transverse jackshaft which drives the input member of a chain and sprocket reduction drive. The output of the reduction drive is coupled to one end of an axle on which are located the drive track drive sprocket wheels.
The drive pulley includes centrifugal actuators through which the drive ratio of the drive pulley is varied progressively as a function of the engine speed. The centrifugal actuators are connected to a movable sheave of the drive pulley. The drive pulley also includes a fixed sheave which is axially fixed. The fixed sheave and the movable sheave are rotatable together. The movable sheave is movable axially toward the fixed sheave by the action of the centrifugal actuators and away from the fixed sheave by a biasing spring. The centrifugal actuators generally consist of centrifugal weights in the form of adjusting arms. Each of the arms is connected to the movable sheave of the drive pulley by a pin, and pivots outwards about its corresponding pin. As they pivot, the arms are in contact with corresponding rollers disposed on a spider fixed relative to the fixed sheave. When the adjusting arms pivot outwards as a result of centrifugal force, they slide against their corresponding roller and the axially movable sheave is pushed towards the fixed sheave.
Due to manufacturing tolerances and the type of connection used, it is possible that the spider and movable sheave can rotate slightly relative to one another during acceleration and deceleration of the drive pulley. As a result, the adjusting arms move slightly in a direction generally parallel to an axis of rotation or their corresponding rollers. This is sometimes referred to as backlash. This slight movement causes rubbing of the adjustable arms against their respective rollers and can result in portions of the arms, the rollers or both to wear and form a flat portion or a recess. In the case of worn surfaces of the arms, the way in which the movable sheave is moved by the arms in response to the speed of rotation of the drive pulley is negatively affected. In the case of worn surfaces of the rollers, it is possible that once the worn surface of a roller makes contact with its corresponding arm, the roller stops rolling, thereby further rubbing against the arm and exacerbating the problem.
Therefore, there is a need for a drive pulley that reduces or eliminates relative rotation between the spider and the movable sheave to help prevent wear of the centrifugal actuators.
In some implementations, the fixed sheave is mounted on a fixed sheave shaft, the movable sheave is mounted on a movable sheave shaft, and the spring biasing the movable sheave away from the fixed sheave is disposed radially between the fixed and movable sheave shafts. In order to reduce friction between the two shafts one or more low friction bushings are disposed radially between the shafts. However, due to the presence of the spring radially between the two shafts, the maximum length of the bushings is limited, which can limit the life of the bushings.
Therefore, there is a need for a drive pulley having a connection between the parts thereof that permit relatively easy displacement of the movable sheave relative to the fixed sheave in an axial direction, while allowing the length of the bushing(s) to be selected in order to provide a desired durability to friction ratio.